The objective of this project is to examine the physiological and pharmacological mechanisms involvedin the centrifugal regulation of primary afferent input. This will be pursued at three levesl ofprimary afferent function. 1. the regulation of afferent terminal polarization by neuronal systems in the dorsal horn with respect to various putative transmitters present in this structure will be examined. 2. The function of the psontaneous antidromic discharge that is generated in some afferent neurons will be evaluated with respect to affects on orthodromic action potential traffic. 3. The effects of antidromic invasion of sensory receptors on their subsequent sensitivity will also be investigated. The investigation will be pursued on the level of single unit activity in dorsal root filaments recorded from the spinal or decerebrate cat or rat preparation. Putative transmitter on chemical mediator functions will be investigated by pharmacological manipulation through the use of agonists and antagonists.